Method of continuous casting thin steel strip

ABSTRACT

In a twin roll continuous caster, the formation of skulls in the triple point region, the heat flux between the molten metal in the casting pool and the surfaces of the casting rolls, and consequently the casting speed and strip thickness, are controlled by controlling the level of carbon dioxide to at least 20% present in the casting area atmosphere above the casting pool supported on the casting surfaces of counter-rotating casting rolls. The carbon dioxide level in casting area may be more than 40%, 50%, 60%, 75% or 90%. The gas mixture in the casting area above the casting pool may be more than 0.05% free oxygen.

RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 61/560,959 filed Nov. 17, 2011, and U.S.Provisional Patent Application No. 61/652,292 filed May 28, 2012, thedisclosures of which are incorporated herein by reference.

BACKGROUND AND SUMMARY

This invention refers to continuous casting of thin steel strip in atwin roll caster.

In a twin roll caster, molten metal is introduced between a pair ofcounter-rotated horizontal casting rolls which are internally cooled sothat metal shells solidify on the moving roll surfaces and are broughttogether at the nip between them to produce a thin strip product,delivered downwardly from the nip between the casting rolls. The term“nip” is used herein to refer to the general region at which the castingrolls are closest together. The molten metal may be received from aladle through a metal delivery system comprised of a tundish and a corenozzle located above the nip, to form a casting pool of molten metalsupported on the casting surfaces of the rolls above the nip andextending along the length of the nip. This casting pool is usuallyconfined between refractory side dams held in sliding engagement withthe end surfaces of the casting rolls so as to restrict the two ends ofthe casting pool against outflow. In the past, the atmosphere in thecasting area, or chamber, above the molten metal in the casting pool wascontrolled by delivering an inert gas such as argon or nitrogen to thearea above the casting pool.

When casting steel strip in a twin roll caster, the thin cast stripleaves the nip at temperatures in the order of 1400° C. or above. Anenclosure is provided beneath the casting rolls to receive the hot caststrip, through which the strip passes away from the strip caster in anatmosphere that inhibits oxidation of the strip. The oxidationinhibiting atmosphere may be created by delivering a non-oxidizing gas,for example, an inert gas such as argon or nitrogen, in the enclosurebeneath the casting rolls. Alternatively, or additionally, the enclosuremay be substantially sealed against ingress of an ambientoxygen-containing atmosphere during operation of the strip caster, andthe oxygen content of the atmosphere within the enclosure may be reducedby oxidation of the strip to remove oxygen from the enclosure asdisclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.

During operation, parameters including the metal flow rate and moltenmetal temperature are controlled which reduce the formation ofsolidified steel skulls in the casting pool in the area where the sidedams, casting rolls and meniscus of the casting pool intersect, i.e. the“triple point” region. These unwanted solidified steel skulls, alsoknown as “snake eggs” in casting, may form from time to time near theside dam and adjacent the end of the delivery nozzle, and can drop intothe cast strip through the casting roll nip. When these skulls dropbetween the roll nip, they may cause the two solidifying shells at thecasting roll nip to “swallow” additional liquid metal between theshells, and may cause the strip to reheat and break disrupting thecontinuous production of coiled strip. Dropped skulls, or snake eggs,may also be detected as visible bright bands across the width of thecast strip, as well as by spikes in the lateral force exerted on thecasting rolls as they pass through the roll nip. Such resistive forcesare exerted against the side dams in addition to the forces generated bythe ferrostatic head in the casting pool. Additionally, skulls resultingin snake eggs in the cast strip passing through the nip between thecasting rolls can cause lateral movement of the casting rolls and theside dams. To resist the increased forces generated, bias forces havebeen applied to the side dams, increasing the force the side dams exerton the ends of the casting rolls, and in turn increasing side dam wear.There remains, therefore, a need to control the formation of unwantedsolidified skulls in the casting pool and formation of snake eggs in thethin metal strip.

In addition, a high heat flux is necessary to achieve high cooling ratesto form shells over the casting surfaces of the casting rolls. Thehigher the heat flux between the molten metal in the casting pool andthe surface of the casting rolls, the larger the degree of cooling ofthe molten steel on the surface of the casting rolls. In turn, suchcontrol of the heat flux between the molten metal in the casting pooland the casting surfaces of the casting rolls provides for the controlof the cast thickness. Such degree of control of heat flux onsolidification of the metal shells on the casting surfaces is desired tocontrol the formation of the steel strip.

We have found that the heat flux from the molten steel in the castingpool to the casting surfaces of the casting rolls, the cast thickness,and the formation of unwanted skulls in the casting pool may becontrolled by providing controlled carbon dioxide levels in the castingarea above the casting pool of molten metal. In addition, carbon dioxidemay be introduced through a gas header onto the casting roll surfacesbetween roll cleaning brushes and the 12 ‘o’ clock position above thecasting rolls as part of the texture gases as described in U.S. Pat. No.7,299,857.

Presently disclosed is a method of casting thin strip comprising thesteps of: assembling a pair of counter-rotating casting rolls laterallyforming a nip between circumferential casting surfaces of the rollsthrough which the metal strip may be cast; assembling a metal deliverysystem above the casting rolls delivering molten metal forming a castingpool supported on the casting surfaces of the casting rolls above thenip; providing above the casting pool an enclosure forming a castingarea above the casting rolls; delivering a gas mixture comprising atleast 20% carbon dioxide to the casting area restricting ingress of airinto the enclosure; and counter-rotating the casting rolls such that thecasting surfaces of the casting rolls each travel inwardly toward thenip to produce a cast strip downwardly from the nip. In one alternative,gas mixture in the casting area above the casting pool comprises morethan 0.05% free oxygen.

The gas mixture in the enclosure above the casting pool may comprisemore than 40% carbon dioxide. Alternatively, the gas mixture maycomprise more than 50% carbon dioxide, more than 60% carbon dioxide, ormore than 75% carbon dioxide. In another alternative, the gas mixturemay comprise greater than 90% carbon dioxide. In any case, the gasmixture may further comprise one or more gases selected from the groupconsisting of nitrogen, argon, hydrogen, helium, water vapor, dry air,and carbon monoxide.

In some alternatives, assembling the casting rolls further comprisesassembling a carbon seal laterally above each casting roll restrictingingress of air into the enclosure. The flow rate of the delivered gasmixture may be configured to provide a positive pressure in theenclosure to restrict the ingress of ambient air.

The gas mixture may be delivered from above the casting pool. The methodmay further comprise varying the gas mixture flow rate to achievedesired properties of the gas layer over the casting pool duringcasting. In any case, the delivery of the gas mixture may notsubstantially disturb the surface of the casting pool. Additionally, oralternatively, the method may further comprise the step of varying thecomposition of the gas mixture to achieve desired properties of thelayer over the casting pool. Nitrogen gas in the enclosure may belimited to control the nitrogen content in the cast strip to a desiredamount.

The gas mixture may form a gas layer over the casting pool between thecasting surfaces of the casting rolls. In one alternative, the gasmixture may be delivered from above the casting pool. In anotheralternative, the gas is delivered to each meniscus near the end portionsof each casting roll. In some embodiments, the gas mixture may bedelivered to the casting area over the casting pool via core nozzlesupport plates, delivering the gas mixture to the enclosure above thecasting pool along the enclosure, and/or from outlets positioned abovethe casting pool. Alternatively, or additionally, the gas mixture may bedelivered from substantially near the edges of the casting pool.

Also disclosed is an apparatus for continuously casting metal stripcomprising a pair of counter-rotatable casting rolls having castingsurfaces laterally positioned forming a nip therebetween through whichthin cast strip can be cast, and on which a casting pool of molten metalcan be formed supported on the casting surfaces above the nip; a metaldelivery system above the casting rolls to deliver molten metal formingthe casting pool supported on the casting surfaces of the casting rollsabove the nip; an enclosure forming a casting area above the castingrolls; a gas delivery system to deliver a gas mixture comprising atleast 20% carbon dioxide to the casting area restricting ingress of airinto the enclosure.

In the alternative, the gas mixture delivered to the casting area in thechamber may comprise more than 40% carbon dioxide. Alternatively, thegas mixture may comprise more than 50% carbon dioxide, more than 60%carbon dioxide, or more than 75% carbon dioxide. In another alternative,the gas mixture may comprise greater than 90% carbon dioxide. In anycase, the gas mixture in the casting area above the casting pool maycomprise more than 0.05% free oxygen. In an alternative, the gas isdelivered to each meniscus near the end portions of each casting roll.The gas mixture may further comprise of one or more gases selected froma group consisting of nitrogen, argon, hydrogen, helium, water vapor,dry air, carbon dioxide and carbon monoxide. The gas delivery system maycomprise at least one gas delivery outlet positioned above the castingpool. Additionally, or alternatively, the gas delivery system maycomprise at least one gas delivery outlet positioned substantially nearthe edge of the casting pool, adjacent where the surface of the castingpool meets the surface of the casting rolls (generally referred to asthe meniscus). The nitrogen in the enclosure may be limited to controlthe nitrogen content in the cast strip to a desired amount.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Patent and Trademark Office uponrequest and payment of the necessary fee.

FIG. 1 is a side elevation view illustrating a continuous twin rollcaster system,

FIG. 2 is a partial side elevation view of a portion of the continuoustwin roll caster system shown in FIG. 1,

FIG. 3 is a partial sectional view through casting rolls shown in FIG.1,

FIG. 4 is a graph showing the relationship between the carbon dioxidelevel in the casting area and the hydrogen level in the casting area andthe free oxygen level in the casting area,

FIG. 5 is a graph showing the relationship between the carbon dioxidelevel in the casting area and drive-side roll force and work-side rollforce during casting, and

FIG. 6 is a graph showing the relationship between the carbon dioxidelevel in the casting area and the concurrently measured data of heatflux, casting speed, and cast thickness taken over time.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1 through 3, a twin roll caster denoted generallyas 11 comprises a pair of laterally positioned casting rolls 22 forminga nip 15 between circumferential casting surfaces of the rolls, forwhich molten metal is delivered from a ladle 23 through a metal deliverysystem 24 to the caster. The metal delivery system 24 comprises atundish 25, a movable tundish 26 and one or more core nozzles 27, shownin FIG. 3, positioned between the casting rolls 22 above the nip 15. Themolten metal delivered to the casting rolls is supported in a castingpool 16 on the casting surfaces of the casting rolls 22 above the nip15. The casting pool 16 of molten steel supported on the casting rolls22 is confined at the ends of the casting rolls 22 by a pair of sidedams 35.

The tundish 25 is fitted with a lid 28. Molten steel is introduced intothe tundish 25 from ladle 23 via an outlet shroud 29. The tundish 25 isfitted with a slide gate 34 to selectively open and close the outlet 31and effectively control the flow of metal from the tundish to theremovable tundish 26. The molten metal flows from tundish 25 throughoutlet 31, and inlet 32 of a distributor 26 (also called the removabletundish or transition piece), through passageways 5, and then todelivery nozzle or core nozzles 27. The core nozzles 27 are supported inthe casting position by a core nozzle support plate 84. The core nozzlesupport plate 84 is positioned beneath the distributor 26 and has acentral opening 88 to receive the core nozzle 27. The core nozzle 27 maybe provided in two or more segments, and at least a portion of each corenozzle segment may be supported by the core nozzle plate 84.

In operation, molten metal is received from the distributor, orremovable tundish 26, through the passageway 5 into the delivery nozzle27. Several passageways 5 may be provided along the length of thedelivery nozzle 27 to provide for a more even flow of molten metal intothe delivery nozzle 27. The molten metal may flow through the deliverynozzle 27 to the outlets 20, through passages 18. The outlets 20 directflow of molten metal to discharge the molten metal into a casting pool16 supported on the surface of the casting rolls 22 above the nip 15.The upper surface 16 a of casting pool 16 (generally referred to as the“meniscus”) will generally rise above the lower end of the deliverynozzle 27 so that the lower end of the delivery nozzle 27 is submergedwithin the casting pool 16.

At the start of a casting operation a short length of imperfect strip istypically produced as the casting conditions stabilize. After casting isstarted, the casting rolls 22 are moved apart slightly and then broughttogether again to cause the leading end of the strip to break away so asto form a clean head end of the following cast strip to start thecasting campaign. The imperfect strip material is dropped into a scrapbox receptacle 40 located beneath caster 11 forming part of theenclosure 10 as shown in FIG. 1.

The casting rolls 22 may typically be about 500 millimeters in diameter,and may be up to 1200 millimeters or more in diameter. The length of thecasting rolls 22 may be up to about 2000 millimeters, or longer, inorder to enable production of strip product of about 2000 millimeters inwidth, or wider, as desired in order to produce strip productapproximately the width of the rolls. Formed in each casting roll 22 isa series of cooling water passages to supply water cooling the castingrolls 22 so that the shells solidify on the casting surfaces 60 as thecasting surfaces move in contact with the casting pool 16. The castingsurfaces may be textured, for example, with a random distribution ofdiscrete projections as described and claimed in U.S. Pat. No.7,073,365.

As the casting rolls 22 are counter-rotated, shells are formed on thecasting surfaces of the casting rolls 22 and are brought together at thenip 15 to produce a solidified thin cast product 12 cast downwardly fromthe nip 15. With reference to FIG. 1, the thin cast strip 12 is passedinto a sealed enclosure 10 and onto a guide table 13, which guides thestrip to a pinch roll stand 14 through which it exits the sealedenclosure 10. The enclosure 10 may not be completely sealed, butappropriately sealed to allow control of the atmosphere within theenclosure so as to restrict ingress of oxygen within the enclosure 10.After exiting the sealed enclosure 10, the strip may pass throughadditional sealed enclosures and pinch rolls to provide tension on thestrip during in-line hot rolling and cooling treatment before coiling.

As shown in FIG. 3, a pair of roll brush apparatus 62 are disposedadjacent the pair of casting rolls 22 such that they may be brought intocontact with the casting surfaces 60 of the casting rolls 22 at oppositesides of nip 15 prior to the casting surfaces 60 of the casting rolls 22coming into contact with the molten metal in casting pool 16 at themeniscus 16 a. Each brush apparatus 62 may comprise a brush frame 64which carries a main cleaning brush 66, for cleaning the castingsurfaces 60 of the casting rolls 22 during the casting campaign asdescribed in U.S. Pat. No. 7,299,857. Optionally in addition, separatesweeper brushes (not shown) for cleaning the casting surfaces of thecasting rolls at the beginning and end of the campaign may also beprovided as shown in U.S. Pat. No. 7,938,164.

Referring to FIG. 3, an enclosure 65 forming a casting area above thecasting pool 16 is bounded by the casting surfaces 60 of the castingrolls 22 above the nip 15, and the side dams 35. The enclosure 65 mayinclude a pair of carbon seals 80, one positioned between the corenozzle support plate 84 and each casting roll 22 restricting ingress ofambient air into the casting area. A gas mixture may be delivered intothe enclosure 65 forming a protective gas layer over the casting pool 16between the casting surfaces 60 of the casting rolls 22. The gas mixturemay be delivered along passageways within the core nozzle support plate84 to the enclosure 65, to one or both sides of the casting nozzle 27.The enclosure 65 may be sealed or semi-sealed, restricting outsideatmosphere gases from entering the enclosure 65. The gas mixture may beintroduced to the enclosure 65 over the casting pool 16 via core nozzleplates 84. As described in U.S. Pat. No. 7,938,164, the side dams 35 maybe positioned on a core nozzle support plate 84 mounted on a rollcassette so as to extend horizontally above, and adjacent the ends of,the casting rolls 22. The core-nozzle plate 84 has a central opening 88to support the metal delivery nozzle 27. The core-nozzle plates 84 maycomprise gas delivery ports 86 located on each side of the castingapparatus 11 such as to deliver a gas mixture into the enclosure 65above the casting pool 16. The gas may be delivered by gas deliveryports 86, positioned at intervals along the length of the core-nozzleplates 84 to provide a more even distribution of the gas mixture alongthe length of the enclosure 65. The gas mixture may be deliveredupwardly into the enclosure 65 such as to avoid disturbing the surface16 a of the casting pool 16, which may cause surface defects in the formof meniscus marks on the surface of the formed thin strip 12. In thealternative, the gas mixture may be delivered from substantially nearthe edges of the casting pool 16 where surface 16 a of the casting pool16 meets the casting surface 60 of casting rolls 22, or directeddownwardly toward the surface 60 of the casting rolls 22. In addition,the gas mixture may be delivered from a gas header 45. The gas header 45may be positioned to deliver gas to the casting surfaces 60 of thecasting rolls 22, at any position between the main cleaning brushes 66and the 12 ‘o’ clock position above the casting rolls 22 as part of thetexture gases, such as at the position indicated by gas header 46.

The gas mixture delivered to the enclosure 65 may comprise at least 20%carbon dioxide forming a layer over the casting pool 16. The castingrolls 22 are counter-rotated such that the casting surfaces 60 of thecasting rolls 22 each rotate inwardly toward the nip 15 and produce athin strip cast downwardly from the nip 15. In one embodiment, the gasmixture delivered to the enclosure 65 may comprise more than 20% carbondioxide. In other embodiments, the gas mixture delivered to the chamber65 may comprise greater than 40%, 50%, 60%, 75%, or 90% carbon dioxide.In each embodiment, the gas mixture may further comprise one or more ofnitrogen, argon, hydrogen, helium, water vapor, dry air, and carbonmonoxide. Alternatively, in each embodiment, the gas mixture may furthercomprise one or more of nitrogen, hydrogen, or air. The desired gasmixture composition may be varied to achieve desired properties of thelayer over the casting pool 16 during casting. The gas mixture flow ratemay be varied to achieve desired properties of the layer over thecasting pool 16 during casting and desired properties and desiredparameters in casting thin strip. The flow rate of the delivered gasmixture may be generally provided to provide a positive pressure withinthe enclosure 65 of 0.14 inches water gauge to restrict the ingress ofambient air into the enclosure 65. The amount of gas required to achievea positive pressure in the enclosure 65 varies with the length of thecasting rolls. A positive pressure may be provided by a flow ratebetween 100 and 200 cubic meters per hour, such as 150 cubic meters perhour in some embodiments.

It has been found that skulls (portions of solid metal) form in thecasting pool 16 adjacent to the casting roll 22 ends and apply resistiveforces against side dams 35 adjacent to the ends of the casting rolls22. Skulls may form in the casting pool 16, along the side dam/castingroll interface in a region known as the triple point, due to the higherrate of heat loss attributed to the triple point region. To resist theincreased forces generated by the skulls, higher forces are needed to bemaintained on the side dams 35 against the casting rolls 22. Theseadditional forces may cause additional wear to the side dams 35, and ifsevere can cause strip break.

In addition, providing the gas mixture to the enclosure 65 withcarbon-dioxide as a substantial or sole component may reduce thenitrogen pick-up by the molten metal in the casting pool 16 and in turnthe cast steel strip. Limiting the amounts of nitrogen content by thepresent process has the added benefit of providing cast strip withreduced nitrogen content. This is done by limiting the amount ofnitrogen in the gas mixture provided in the enclosure 65 during casting,allowing the continuous caster 11 to produce a cast strip 12 withreduced levels of nitrogen between 25 and 75 ppm or lower.

FIG. 4 sets forth graphs showing the correlation between the carbondioxide level and the level of hydrogen 87 and oxygen 89 in theenclosure 65 above the casting pool 16. During testing, the levels ofcarbon dioxide 90, hydrogen 87 and oxygen 89 are measured at discreteinstances, creating the stepped graphs shown in FIG. 4. Through testing,a gas mixture comprising approximately 50% carbon dioxide gas wasintroduced into the enclosure 65 above the casting pool 16 on eitherside of the core nozzle 27, as illustrated between markers 102 and 103,the level of hydrogen 87 was reduced from approximately 0.075% to 0.100%to approximately 0.040 to 0.015%. Furthermore, we also found that thelevel of free oxygen 89, in the enclosure 65, was above 0.05% to betweenabout 0.055% and 0.075%, as shown, when the gas mixture delivered to theenclosure 65 comprised of approximately 50% carbon dioxide. When theintroduction of carbon dioxide into the enclosure 65 ceased, representedat marker 103, the levels of hydrogen 87 and free oxygen 89 returned totheir previous levels. During the test illustrated by FIG. 4, carbondioxide gas was reintroduced into the enclosure 65 above the castingpool 16, the time of introduction illustrated by marker 104. Atapproximately the same time the casting nozzle 27 started to break upcausing debris to fall through the nip 15 between the casting rolls 22,providing inconsistent data. However, as can be seen in the graphs ofFIG. 4, hydrogen levels 87 fell as previously, to approximately 0.040%to 0.015%. Additionally, the level of free oxygen 89 also increased, asseen previously, to at least 0.05% to 0.07% to 0.08% as shown.

Through testing, we have found that the addition of carbon dioxide inthe chamber 65 above the casting pool 16 decreases the formation ofskulls, and, in turn, snake eggs in the cast strip 12. The presence ofskulls is detected by the lateral forces they exert on the casting rolls22 as they pass between them at the nip 15. Skulls also cause visiblebright bands, i.e., snake eggs, to be formed across the width of thestrip, which are defects in the surface of the cast strip. Duringtesting, the presence of snake egg forming skulls was monitored bymeasuring the drive-side (DS) casting roll force 92 (Newtons) and thework-side (WS) casting roll force 94 (Newtons).

FIG. 5 sets forth graphs showing correlation between the level of carbondioxide 90 in the gas mixture, the drive-side casting roll force 92, andthe work-side casting roll force 94 measured over time. During the testillustrated in FIG. 5, carbon dioxide gas 90 was introduced into theenclosure 65 above the casting pool 16, on both sides of the castingnozzle 27, lasting for approximately thirty minutes. The period in whichcarbon dioxide gas 90 was introduced into the enclosure 65 above thecasting rolls 22 is represented by the area of the graph between markers102 and 103. When no carbon dioxide is delivered to the enclosure 65above the casting pool 16, both the drive-side casting roll force 92 andthe work-side casting roll force 94 show peaks 93 in excess of 9000N.Each peak 93 represents one or more skulls dropping and travellingthrough the nip 15 of the casting rolls 22, causing snake eggs, andexerting a lateral pressure on the casting rolls 22, measured by a forcedetector. Once the amount of carbon dioxide 90 introduced to the chamber65 above the casting rolls 16 was increased, represented at marker 102,the incidence and the size of the peaks 93 in both the drive-sidecasting roll force 92 and the work-side casting roll force 94,substantially decreased, indicating that snake egg forming skulls wereinhibited in the triple point region and therefore were not fallingbetween the casting rolls 22. Conversely, once the carbon dioxide level90 in the gas mixture, delivered to the chamber 65, was decreased backto original levels, represented at marker 103, both the drive-sidecasting roll force 92 and the work-side casting roll force 94 showedincreased incidence of peaks, with the peaks again reaching in excess of9000N (indicating that the formation of skulls had once againcommenced). A force of more than 9000N on the drive-side casting roll,or work-side casting roll, may be expected to cause strip breakage.

FIG. 5 also illustrates, at marker 104, when the level of carbon dioxide90 in the enclosure 65 above the casting pool 16 was again increased byintroducing a gas mixture of carbon dioxide gas for example 40% to 50%through ports 86 in the casting nozzle support plates 84. This portionof the graphs illustrate that the incident rate of peaks 93, in thedrive-side casting roll force 92 and the work-side casting roll force94, reaching in excess of 9000N, is greatly reduced compared with theareas of the graph which represent no introduction of carbon dioxidegas. The peaks 93 in the area of the graph beyond marker 104 reaching inexcess of 9000N are as a result of the casting nozzle 27 breaking up anddebris from the casting nozzle 27 falling through the nip 15 between thecasting rolls 22 causing spikes in both drive-side casting roll force 92and work-side casting force 94.

The results of testing, illustrated in FIG. 5, demonstrate that theaddition of gas mixture comprising carbon dioxide gas above 20%, forexample 40% to 50%, into the enclosure 65 above the casting pool 16significantly decreased the incident rate of spikes in the force exertedon both the casting rolls 22, thus revealing the reduction of snake eggforming skulls travelling through the nip 15 of the casting roll 22. Theaddition of a gas mixture containing carbon dioxide above 20% asindicated greatly reduced the formation of skulls in the casting pool 16in the triple point region adjacent the side dams 35. Thus it has beenfound that controlling the level of carbon dioxide into the atmosphereabove the casting pool 16 substantially improved the quality of thestrip cast from the continuous casting apparatus 11.

As previously explained, when casting steel strip in a twin roll caster,the molten metal in the casting pool will generally be at a temperatureof the order of 1500° C. and above. A high heat flux between the moltenmetal and the casting surface 60 of the casting rolls 22 is necessary toachieve the high cooling rates required to solidify the molten metalinto shells on the casting surface 60 and form cast strip at the nip 15.Testing has revealed a correlation between the indicated level of carbondioxide in the chamber 65 above the casting pool 16 and the amount ofheat flux from the molten metal in the casting pool 16 to the castingrolls 22.

As illustrated in FIG. 6, when carbon dioxide gas was delivered to thecasting area 65 above the casting pool 16, as illustrated betweenmarkers 102 and 103, the heat flux 96 from the molten steel in thecasting pool 16 to the casting rolls 22 substantially increased. FIG. 6sets forth graphs showing test results for a gas mixture containingapproximately 50% carbon dioxide. With a carbon dioxide level 90 of 50%in the casting area 65 above the casting pool 16, the heat flux 96between the molten steel in the casting pool 16 and the surface 16 a ofthe casting rolls 16 increased by 10 to 20%. The correlation between theamount of carbon dioxide in a gas mixture delivered to the chamber 65above the casting pool 16 illustrates that the presently disclosedmethod of casting steel strip provides a sensitive direct control of theheat flux 96 between the molten metal in the casting pool 16 and thecasting surfaces 60 of the casting rolls 22.

Moreover, when the carbon dioxide level 90 in the casting area 65 abovethe casting pool 16 is increased and heat flux 96 from the molten metalin the casting pool 16 to casting surfaces 60 of the casting rolls 22correspondingly increases, the casting speed may be increased or stripthickness may be increased, or both. A higher heat flux 96 between themolten metal in the melt pool 16 and the surface 60 of the casting rolls22 increases the rate at which the molten metal solidifies into shellson the casting roll surface 60. Maintaining a constant casting speedwould result in forming a thicker cast strip, while increasing thecasting speed will maintain the thickness of the product. The castingspeed 95 is a variable which can be controlled by the operator of thecontinuous casting apparatus 24.

As demonstrated in FIG. 6, when the level of carbon dioxide introducedinto the casting area 65 above the casting pool 16 increased toapproximately 50%, the operator had to increase the casting speed 95 tomaintain a constant cast strip thickness 98. As shown between markers102 and 103, representing the introduction of carbon dioxide gas above20% as indicated into the enclosure 65 above the casting pool 16, theoperator increased the casting speed 96 from approximately 60 meters perminute (m/min) to between 65 m/min and 70 m/min in order to maintain acast strip thickness 98 of approximately 1.9 mm. Subsequently, after theintroduction of carbon dioxide gas into the enclosure 65 above thecasting pool 16 was stopped, the operator had to decrease the castingspeed 95 in order to maintain the cast strip thickness 98 ofapproximately 1.9 mm. The graphs set forth in FIG. 6 illustrate thatwhen carbon dioxide gas above 20% as indicated was reintroduced into theenclosure 65 above the casting pool 16, at marker 104, the operatoragain had to increase the casting speed 95 to maintain a cast stripthickness 98 of about 1.9 mm. However, shortly thereafter, as discussedabove, the casting nozzle 27 began to break up causing debris to fallthrough the nip 15 between the casting rolls 22 creating forces betweenthe casting rolls 22 in excess of 9000N, causing strip break andpreventing a constant cast strip thickness 98 from being maintained, andthe ensuring data shown in FIG. 6.

As a result, through testing we have found that modifying the heat flux96 between the molten metal in the casting pool 16 and the casting rollsurface 60 in turn enables increases in the casting speed 95 and/or thestrip thickness 98. Consequently, the strip thickness, the castingspeed, and the quality of the cast strip product may be controlled bycontrolling the level of carbon dioxide introduced into the casting area65 above the casting pool 16.

The presently disclosed method of casting steel strip and apparatus forcontinuously casting metal strip provide for the control of snake eggsformation, heat flux, casting speed, and cast thickness by controllingthe level of carbon dioxide in the casting area above the casting pool.

While the invention has been described with reference to certainembodiments and alternatives it will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from itsscope. Therefore, it is intended that the invention not be limited tothe particular embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method of casting thin strip comprising thesteps of: assembling a pair of counter-rotating casting rolls laterallyforming a nip between circumferential casting surfaces of the rollsthrough which the metal strip may be cast; assembling a metal deliverysystem above the casting rolls delivering molten metal forming a castingpool supported on the casting surfaces of the casting rolls above thenip; providing above the casting pool an enclosure forming a castingarea above the casting rolls; delivering a gas mixture comprising atleast 20% carbon dioxide to the casting area restricting ingress of airinto the enclosure; and counter-rotating the casting rolls such that thecasting surfaces of the casting rolls each travel inwardly toward thenip to produce a cast strip downwardly from the nip.
 2. The method ofcasting thin strip as claimed in claim 1 wherein the gas mixture in thecasting area above the casting pool comprises more than 0.05% freeoxygen.
 3. The method of casting thin strip as claimed in claim 1wherein the gas mixture comprises more than 40% carbon dioxide.
 4. Themethod of casting thin strip as claimed in claim 1 wherein the gasmixture comprises more than 50% carbon dioxide.
 5. The method of castingthin strip as claimed in claim 1 wherein the gas mixture comprises morethan 60% carbon dioxide.
 6. The method of casting thin strip as claimedin claim 1 wherein the gas mixture comprises more than 75% carbondioxide.
 7. The method of casting thin strip as claimed in claim 1wherein the gas mixture comprises greater than 90% carbon dioxide. 8.The method of casting thin strip as claimed in claim 1 wherein the gasmixture further comprises one or more gases selected from a groupconsisting of nitrogen, argon, hydrogen, helium, water vapor, dry air,carbon dioxide and carbon monoxide
 9. The method of casting thin stripas claimed in claim 1 wherein the step of assembling the casting rollsfurther comprises: assembling a carbon seal laterally above each castingroll restricting ingress of air into the enclosure.
 10. The method ofcasting thin strip as claimed in claim 1 wherein the gas mixture isdelivered from above the casting pool.
 11. The method of casting thinstrip as claimed in claim 1 wherein the gas mixture is delivered fromsubstantially near the edges of the casting pool.
 12. The method ofcasting thin strip as claimed in claim 1 further comprising the step ofvarying the gas mixture flow rate to achieve desired properties of thegas layer over the casting pool during casting.
 13. The method ofcasting thin strip as claimed in 1 further comprising the step ofvarying the composition of the gas mixture to achieve desired propertiesof the layer over the casting pool.
 14. The method of casting thin stripas claimed in claim 1 wherein the delivery of the gas mixturesubstantially does not disturb the surface of the casting pool.
 15. Themethod of casting thin strip as claimed in claim 1 wherein the flow rateof the delivered gas mixture is configured to provide a positivepressure in the enclosure to restrict the ingress of ambient air. 16.The method of continuously casting metal strip as claimed in claim 1where nitrogen gas in the enclosure is limited to control the nitrogencontent in the cast strip to a desired amount.
 17. A method ofcontinuously casting metal strip as claimed in claim 1 where the gas isdelivered to each meniscus near the end portions of each casting roll.18. An apparatus for continuously casting metal strip comprising: a pairof counter-rotatable casting rolls having casting surfaces laterallypositioned forming a nip therebetween through which thin cast strip canbe cast, and on which a casting pool of molten metal can be formedsupported on the casting surfaces above the nip; a metal delivery systemabove the casting rolls to deliver molten metal forming a casting poolsupported on the casting surfaces of the casting rolls above the nip; anenclosure forming a casting area above the casting rolls; and a gasdelivery system to deliver a gas mixture comprising at least 20% carbondioxide to the casting area restricting ingress of air into theenclosure.
 19. The apparatus for continuously casting metal strip asclaimed in claim 18 wherein the gas mixture in the casting area abovethe casting pool is comprises more than 0.05% free oxygen.
 20. Theapparatus for continuously casting metal strip as claimed in claim 18wherein the gas mixture comprises more than 40% carbon dioxide.
 21. Theapparatus for continuously casting metal strip as claimed in claim 18wherein the gas mixture comprises more than 50% carbon dioxide.
 22. Theapparatus for continuously casting metal strip as claimed in claim 18wherein the gas mixture comprises more than 60% carbon dioxide.
 23. Theapparatus for continuously casting metal strip as claimed in claim 18wherein the gas mixture comprises more than 75% carbon dioxide.
 24. Theapparatus for continuously casting metal strip as claimed in claim 18wherein the gas mixture comprises greater than 90% carbon dioxide. 25.The apparatus for continuously casting metal strip as claimed in claim18 wherein the gas mixture further comprises one or more gases selectedfrom a group consisting of nitrogen, argon, hydrogen, helium, watervapor, dry air, carbon dioxide and carbon monoxide.
 26. The apparatus ofclaim 18 further comprising: a carbon seal laterally positioned aboveeach casting roll to restrict oxygen from entering the chamber.
 27. Theapparatus for continuously casting metal strip as claimed in claim 18wherein the gas delivery system further comprises at least one gasdelivery outlet positioned above the casting pool.
 28. The apparatus forcontinuously casting metal strip as claimed in claim 18 wherein the gasdelivery system further comprises at least one gas delivery outletpositioned substantially near the edge of the casting pool.
 29. Theapparatus for continuously casting metal strip as claimed in claim 18where nitrogen gas in the enclosure is limited to control the nitrogencontent in the cast strip to a desired amount.
 30. The apparatus forcontinuously casting metal strip as claimed in claim 18 where the gas isdelivered to each meniscus near the end portions of each casting roll.